1. Field of the Invention
This invention relates to the field of electronically tunable optical filters utilizing noncollinear acousto-optic interaction in a birefringent crystal.
2. Description of Prior Art
Electronically tunable optical filters have been constructed so that an incident light beam of a first polarization is diffracted by an acoustic wave in a birefringent crystal to shift from the first to a second polarization of the light beam for a selected bandpass of optical frequencies. The center wavelength of the passband of this type of filter is electronically tunable by changing the frequency of the acoustic wave within the crystal.
Two basic types of acousto-optic tunable filters (AOTFs) have been constructed: collinear and noncollinear. In the collinear filter, the incident and diffracted optical waves inside the birefringent crystal are collinear with the acoustic wave. The diffracted light beam at the selected passband is separated from the incident light beam by crossed polarizers. One significant feature of the collinear AOTF is that the narrow filter bandpass can be maintained for incident light having a distribution of incident light directions. This large angular aperture characteristic is important in optical systems applications where a large optical throughput is required. The collinear type of acousto-optic filter is disclosed in an article entitled "Acousto-Optic Tunable Filters," appearing on pages 744-747 in the June, 1969 issue of The Journal of the Optical Society of America (Vol. 59, No. 6), and in U.S. Pat. No. 3,679,288, entitled "Tunable Acousto-Optic Method and Apparatus."
In the noncollinear filter, the optical wave inside the birefringent crystal are noncollinear with the acoustic wave. The diffracted beam at the passband is selected from the incident light beam either by crossed polarizers or by spatial separation. One type of noncollinear AOTF was described in a paper entitled "Noncollinear acousto-optic filter," presented at the 1973 IEEE/OSA conference on laser engineering and applications and in U.S. Pat. Nos. 3,944,334, 3,944,335, and 3,953,107, all of which were entitled "acousto-optic filter." This type of noncollinear AOTF has a small angular aperture and must be restricted to a well-collimated light source. A second type of noncollinear AOTF is disclosed in an article entitled "Noncollinear Acousto-Optic Filter with Large Angular Aperture," appearing on pages 370-372 of the Oct. 15, 1974 issue of the Applied Physics Letters (Vol. 25), and in U.S. Pat. No. 4,052,121 entitled "Noncollinear Tunable Acousto-Optic Filter." In contrast to the first type, this type of noncollinear AOTF has the important feature of large angular aperture. The large angular aperture is due to the proper choices of interaction geometry wherein the tangents to the loci of the incident and diffracted light wave vectors are "parallel," a condition known as non-critical phase matching (NPM). The above two types of noncollinear AOTFs are referred to as the critical phase matching (CPM) and NPM types, respectively.
Due to its large angular aperture, the NPM type of AOTF has been most commonly used. However, the additional "parallel tangents" requirement also introduces practical fabrication difficulties, with the consequent higher costs and lower performance. The CPM type AOTF is suitable for applications involving collimated light sources, such as the wavelength division multiplexing (WDM) in fiberoptic communications network.
The two basic types of AOTF described above, the collinear and noncollinear, are distinguishable from the interaction geometry of the acoustic and optical wave vectors. In a collinear AOTF, the wave vectors of incident and diffracted light and the acoustic wavevector (i.e., phase velocity vector) are all substantially aligned along a principal axis of the birefringent crystal. In a noncollinear AOTF, the directions of all three wave vectors are different and are not along any principal crystal axis. The angle between the incident light and the acoustic wave directions are substantially greater than zero. Due to the different acousto-optic interaction geometry, the characteristics of collinear and noncollinear AOTFs are entirely different. For instance, TeO.sub.2 is the most efficient material for noncollinear AOTFs, but is not applicable to collinear AOTFs since the relevant elasto-optical coefficient is zero. Collinear AOTFs rely on the use of crossed polarizers to separate the filtered light beam from the broadband light beam; while in a noncollinear AOTF, these light beams are separated spatially. Discussion of the two basic types of AOTF can be found, for instance, in a review paper entitled, "Acousto-Optic Tunable Filters", appearing on pages 139 to 159 in Acousto-Optic Signal Processing (N. Berg and J. Lee, ed.), Marcel Dekker, New York, 1983.
The characteristics of both types of AOTFs are dependent on the acoustic wave propagation in the medium. The acoustic phase velocity V.sub.p is along the direction of the acoustic wave vector K.sub.a. In an acoustically isotropic medium the magnitudes of the phase velocities are equal for all directions of propagation. In this case the group velocity V.sub.g, which is along the direction of the acoustic beam or energy flow, is collinear with the phase velocity V.sub.p. On the other hand, in an acoustically anisotropic medium, the magnitudes of the phase velocities along different directions are not equal. As a result, the direction of the acoustic beam or group velocity V.sub.g is in general different from that of the acoustic wave or phase velocity V.sub.p. For wave propagation in some acoustically anisotropic crystals, the angular deviation of the group velocity from the phase velocity, commonly referred to as the acoustic walkoff, can be quite large. For instance, in TeO.sub.2 the slow shear acoustic wave propagating at 15.degree. with the [110] axis exhibits an acoustic walkoff of 57.4 degrees. A detailed discussion of acoustic wave propagation in TeO.sub.2 crystal is described in a paper by Y. Ohmachi et al appearing on pages 164-168 in Volume 51 of the Journal of the Acoustical Society of America, 1972.
The acoustic walkoff limits the total length of a collinear AOTF and results in performance degradation. An improved collinear AOTF with selected crystal orientation for compensating the acoustic anisotropy effect is described in U.S. Pat. No. 3,756,689 entitled, "Electronically Tunable Acousto-Optic Filter Having Selected Crystal Orientation".
Recent development of AOTFs has been focused on the integrated optic or guided wave structures. The integrated optical AOTF is described in an article by Y. Ohmachi and J. Noda, entitled "LiNbO.sub.3 TE-TM mode converter using collinear acousto-optic interaction," IEEE J. Quantum Electron. Vol QE-13, pp. 43-46, 1977. The integrated optic AOTF provides the advantage of low drive power. To date, all integrated optic AOTF work has been restricted to LiNbO.sub.3 using collinear acousto-optic interaction, i.e. the phase velocity of the optical and acoustic waves are collinear. Noncollinear AOTFs have not been explored in the integrated optic configuration primarily because of the difficulty of realizing long interaction length and low drive power. This major deficiency of the non-collinear AOTF may also be resolved by properly choosing a CPM-type configuration.